In the title compound, C12H6N2O2·C2H5OH, the mol­ecule of the 1,10-phenanthroline-5,6-dione is approximately planar, with a maximum deviation of 0.051 (1) Å. In the crystal, mol­ecules are linked by O—H⋯N and weak C—H⋯O hydrogen bonds, forming supra­molecular chains propagating along [110]. π–π stacking inter­actions are observed between the pyridine rings of neighbouring chains, the centroid–centroid separations being 3.6226 (11) and 3.7543 (11) Å.

The title compound was prepared according to literature method (Paw & Eisenberg, 1997). An ice-cold mixture of concentrated H2SO4 (40 mL) and HNO3 (20 mL) was added to 4 g of 1,10-phenanthroline (0.02 mol) and 4 g of KBr (0.03 mol). The mixture was heated at 90 oC for 3 h. The hot yellow solution was poured over 200 mL of ice and neutralized carefully with sodium hydroxide until neutral to slightly acidic pH. Extraction with CH2Cl2 (4*100 mL) followed by drying with Na2SO4 and removal of solvent gave 2.8 g (yield = 67%) of 1,10-phenanthroline-5,6-dione. This product was purified further by crystallization from ethanol.

Crystal data, data collection and structure refinement details are summarized in Table 1. Carbon-bound H-atoms were placed in calculated positions and were included in the refinement in the riding model approximation with Uiso(H) = 1.5Ueq(C) for methyl H atoms and 1.2Ueq(C) for the others. The hy­droxy H atom was located in a difference Fourier map, and was refined with distance restraints of O—H = 0.84±0.01, Uiso(H) = 1.2Ueq(O).

According to the structural analysis, the bond lengths and angles of the title compound are generally within normal ranges. The asymmetric unit of the title compound consists of one 1,10-phenanthroline-5,6-dione molecule and one ethanol molecule. Between molecules, O—H···N and C—H···O hydrogen bonds can be found that further form one-dimensional chain. The weak π···π stacking inter­actions between adjacent chains are also observed [centroid– centroid separations being 3.6226 (11) and 3.7543 (11) Å].

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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